COMPARATIVE-STUDY OF EMBRYONIC THERMORESISTANCE OF 2 INBRED STRAINS OF THE MEDAKA (ORYZIAS-LATIPES)

By using inbred strains (HO4C and HB32C) of the medaka, Oryzias latipes, the involvement of genetic factor(s) in the determination of thermoresistance of fish was investigated. The thermoresistance of embryos of the medaka was quantitated by the fraction of the embryos surviving 1 day after heat treatment. At early stages of development (st. 13 and st. 20-21), the HO4C strain was more resistant than the HB32C strain. At st. 20-21, the HO4C strain was more resistant than the HB32C strain at all temperatures used (42, 43, and 44-degrees-C). At later stages of development (st. 27 and st. 32), however, the HB32C strain was more resistant than the HO4C strain. The results of genetic cross experiments raised the following possibilities; the thermoresistance of embryos at early developmental stages can be lowered by some factor(s) inherited in the HO4C strain and/or increased by those in the HB32C strain. By contrast, the sensitivity of embryos at later stages of development was not affected by factor(s) of their parents, but by their own genetic constitution.